Wireless communications systems which provide communications through the Earth's atmosphere typically use radio-frequency (RF) communications technologies rather than optical technologies. The main reason is that optical signals become severely attenuated in the atmosphere due to absorption and scattering.
On the other hand, optical signals propagate well in environments which are substantially free of attenuating influences. For example, satellite communications systems are being developed which use laser cross-links between the satellites. The high bit rates attainable with optical communications and the minimal attenuation through free space makes laser communications desirable for such applications.
Both RF and optical communication links through the Earth's atmosphere suffer from attenuation. However, prior art systems have demonstrated that RF attenuation is manageable, even at relatively high bit-rates, by modifying signal transmission levels and modulation techniques. Prior-art optical technologies, on the other hand, have been unsuccessful at overcoming atmospheric attenuation, making high bit-rate optical communications through the atmosphere untenable.
Prior art systems have used optical communication signals at lower bit-rates for certain applications. For example, U.S. Pat. No. 5,038,406 discloses a two-way submarine communication system in which an airborne transceiver sends a down-link pulse-modulated laser signal to a submarine below the ocean's surface. The laser transmitter includes a fast-pulse rejection circuit which filters out pulses shorter than a minimum width (nominally 300 ns). This limits the ability to penetrate aerosols and obstructions because of the wide pulse width.
High frequency laser communications through lossy mediums has not been realizable because of attenuation of laser light in the atmosphere (scattering and absorption) has limited prior art technologies' ability to communicate short pulse width signals. Therefore, what is needed is a method and apparatus which provides short pulse width laser communications through a lossy medium while overcoming attenuation problems due to absorption and scattering.